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Hey!
I really do hope that Bimmerfest will be able to help me with this problem!!!
I own a 1999 Z3 Bella Machina.... and have been struggling with its heater control for quite some time now, even had it with the dealers without success.
When I turn the heater control on, no matter even how slightly, it automatically goes onto full heat.... that is still okay, but then it simply doesn't go off again, and does so in its own time... which can be anything from a week or two, or a month or three.
It is useless to turn the air conditioner on, as the flow of heat is so strong, it simply overrides it.
Once she has decided by herself to stop blowing hot air, the air conditioner works absolutely perfectly.
Firstly, where in the engine can I go to turn the flow of heat off manually, and secondly what can I do to repair it if not even highly skilled Mechanic cannot find the cause, nor a solution to the problem.
Your assistance will be greatly appreciated, as you can very well imagine the discomfort with outside temperatures in the mid 30's and full heat inside the car.
I am looking forward to members' help or advise on this!!
Kind regards
Derek

There are several common causes for this. It could be a broken pin on the back of the heater control knob, it could be a broken bowden cable that goes between the knob and the blend door, it could be a mis-adjustment where the cable connects to the blend door, and/or it could be a stuck blend door.

Near the top of the page, on the right is a button called "Search this Forum." Click on that and search on "heater control" or similar term. You will find many threads on how to diagnose and fix this problem, along with lots of pictures and illustrations.

There is a water valve mounted on the firewall that might be usable to turn off the water to the heater core. It's mounted on the firewall behind the engine. On left-hand drive cars, it's on the driver's side next to the brake master cylinder. It normally turns off the water to the heater at low RPM.
You can go to RealOEM.com to find a picture of it for your model.
I'm not sure, but maybe if you disconnected the wire to the valve, it would be "off" and you can enjoy your car for the rest of the summer. Of course you can re-route the hoses to bypass the heater core, but it is really tight back there and not much fun to work on the hoses.

Sounds like your heater control valve is sticking open. This valve has a solenoid that opens when +12V is applied and closes when voltage is removed, this is how the car regulates the heater output. Sounds like yours opens just fine but then hangs up and doesn't close. Usually they don't stick wide open, typically they partially close and "leak" hot coolant through the heater core and are a common culprit for poor AC performance. Here's a link to the RealOEM page showing the typical Z3 control valve (item 4 in the diagram)http://www.realoem.com/bmw/showparts...03&hg=64&fg=18 You should double check the valve P/N since I didn't know what engine you have. The valve is located on the engine firewall and is simple to replace. It can be bought from any number of aftermarket BMW parts suppliers for a bit less than the price shown on RealOEM.

Before you replace the valve check the connector for +12V while the malfunction is occuring. If there's 12V when there should be none then you need to find and replace the control relay which is even easier and less expensive than replacing the valve.

Looked up the heater control circuit in the service manual and there is no relay, just a fuse, a 2A in panel A605 in the center of the dashboard behind the radio. If you have 12V at the control valve with the temp control in the full hot position then the fuse is good. If the voltage goes to 0 when the control is turned to the full cold position then the problem is definitely the heater control valve.